Studies have been performed in our laboratory to elucidate the effects of surgery upon the circadian rhythm of adrenal corticosteroids. The effects of surgical trauma and hemorrhagic shock were investigated in the rat. When subjected to hemorrhage or surgical trauma, dramatic alterations occurred in the corticosteroid rhythm which persisted up to 72 hours. The female baboon was used in an extensive set of primate studies. Ten animals were studied for control rhythms and were found to have a true circadian rhythm. Seven of these animals were then subjected to either a 20% blood volume hemorrhage or surgical trauma including laparotomy and bowel traction. Both hemorrhage and surgical trauma induced abberations in the normal circadian rhythm of cortisol. With increasing surgical stress, these changes in rhythmicity increased in magnitude and frequency. Using frequent blood sampling techniques, 10 patients were studied for 24 hours before and after surgery. All patients showed an increase in post-operative cortisol levels with an accompanying phase shift in the circadian cycle, which was proportional to the degree of tissue trauma associated with the operation. We conclude that surgery may cause a prolonged disruption of the circadian rhythm of adrenal corticosteroid. These changes may be important to the post-operative management of surgical patients.